1. Field of the Invention
The invention relates generally to a magnetic memory device and a method for manufacturing the memory device. This invention relates more particularly to a magnetic random access memory (MRAM) wherein a memory cell is formed using a magnetic tunnel junction (MTJ) element that stores information “1” or “0” on the basis of a tunneling magneto-resistive (TMR) effect.
2. Description of the Related Art
In these years, many kinds of memories that store information based on new principles have been proposed. One of them is a magnetic random access memory (MRAM) using a tunneling magneto-resistive (TMR) effect. The MRAM is disclosed, for example, in ISSCC2000 Technical Digest, p. 128, Roy Scheuerlein et al., “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel junction and FET Switch in Each Cell”.
FIGS. 22A, 22B, and 22C are cross-sectional views of a magnetic tunnel junction (MTJ) element of a prior-art magnetic memory device. The MTJ element used as a memory element of the MRAM will now be described.
As is shown in FIG. 22A, an MTJ element 30 has such a structure that an insulating layer (tunnel junction layer) 42 is interposed between two magnetic layers (ferromagnetic layers) 41 and 43. In the MRAM, the MTJ element 30 stores information “1” or “0”. The information “1” or “0” is determined on the basis of whether the directions of magnetization of the two magnetic layers 41 and 43 in the MTJ element 30 are parallel or anti-parallel. The term “parallel” in this context means that the directions of magnetization of two magnetic layers 41 and 43 are the same, and “anti-parallel” means that the directions of magnetization of two magnetic layers 41 and 43 are opposite to each other.
Specifically, when the directions of magnetization of two magnetic layers 41 and 43 are parallel, as shown in FIG. 22B, the tunnel resistance of the insulating layer 42 interposed between the two magnetic layers 41 and 43 takes a minimum value. This state corresponds to, for example, “1”. On the other hand, when the directions of magnetization of two magnetic layers 41 and 43 are anti-parallel, as shown in FIG. 22C, the tunnel resistance of the insulating layer 42 interposed between the two magnetic layers 41 and 43 takes a maximum value. This state corresponds to, for example, “0”.
Normally, an anti-ferromagnetic layer 103 is provided on one of the two magnetic layers 41 and 43. The anti-ferromagnetic layer 103 is a member for fixing the direction of magnetization of one magnetic layer 41, thus permitting easy rewriting of information by merely changing the direction of magnetization of the other magnetic layer 43 alone.
FIG. 23 shows MTJ elements arranged in a matrix in a prior-art magnetic memory device. FIG. 24 shows asteroid curves in the prior-art magnetic memory device. FIG. 25 shows hysteresis curves in the prior-art magnetic memory device. The principle of the write operation for the MTJ element will now be described in brief.
As is shown in FIG. 23, MTJ elements 30 are arranged at intersections between write word lines 22 and bit lines (data select lines) 35, which are arranged to cross each other. A data write operation is performed by supplying a current to each of the write word lines 22 and bit lines 35 and setting the directions of magnetization of the MTJ elements 30 in a parallel state or an anti-parallel state, making use of magnetic fields produced by the current flowing in both lines 22 and 35.
For example, in the data write mode, the bit lines 35 are supplied with only a current I1 that flows in one direction, and the write word lines 22 are supplied with a current I2 that flows in one direction or a current I3 that flows in the other direction in accordance with data to be written. When the write word line 22 is supplied with the current I2 that flows in the one direction, the direction of magnetization of the MTJ element 30 is parallel (“1” state). On the other hand, when the write word line 22 is supplied with the current I3 that flows in the other direction, the direction of magnetization of the MTJ element 30 is anti-parallel (“0” state).
How the direction of magnetization of the MTJ element 30 is changed will now be described. When a current is supplied to a selected write word line 22, a magnetic field Hx occurs in a longitudinal direction, i.e. an Easy-Axis direction, of the MTJ element 30. When a current is supplied to a selected bit line 35, a magnetic field Hy occurs in a transverse direction, i.e. a Hard-Axis direction, of the MTJ element 30. As a result, a composite magnetic field of the Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy acts on the MTJ element 30 located at the intersection of the selected write word line 22 and selected bit line 35.
In a case where the magnitude of the composite magnetic field of the Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy is in an outside region (hatched region) of asteroid curves indicated by solid lines in FIG. 24, the direction of magnetization of the magnetic layer 43 can be reserved. On the other hand, when the magnitude of the composite magnetic field of the Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy is in an inside region (blank region) of the asteroid curves, the direction of magnetization of the magnetic layer 43 cannot be reversed.
In addition, as indicated by solid and broken lines in FIG. 25, the magnitude of the Easy-Axis magnetic field Hx, which is necessary for varying the resistance value of the MTJ element 30, varies with the magnitude of the Hard-Axis magnetic field Hy. Making use of this phenomenon, the direction of magnetization of only the MTJ element 30 located at the intersection of the selected write word line 22 and selected bit line 25, among the arrayed memory cells, is altered, and thus the resistance value of the MTJ element 30 can be varied. Oe in FIG. 25 is an abbreviation of “oersted” and unit of a magnetic field.
A variation ratio in resistance value of the MTJ element 30 is expressed by an MR (Magneto-Resistive) ratio. For example, if the magnetic field Hx is produced in the Easy-Axis direction, the resistance value of the MTJ element 30 varies, e.g. about 17%, compared to the state before the production of magnetic field Hx. In this case, the MR ratio is 17%. The MR ratio varies corresponding to the properties of the magnetic layer. At present, MTJ elements with an MR ratio of about 50% have successfully been obtained.
As has been described above, the direction of magnetization of the MTJ element 30 is controlled by varying each of the magnitudes of Easy-Axis magnetic field Hx and Hard-Axis magnetic field Hy and by varying the magnitude of the composite magnetic field of the fields Hx and Hy. In this manner, a state in which the direction of magnetization of the MTJ element 30 is parallel or a state in which the direction of magnetization of the MTJ element 30 is anti-parallel is created, and information “1” or “0” is stored.
FIG. 26 is a cross-sectional view of a prior-art magnetic memory device having a transistor. FIG. 27 is a cross-sectional view of a prior-art magnetic memory device having a diode. An operation of reading out information from the MTJ element will be described below.
Data read-out is effected by supplying a current to a selected MTJ element 30 and detecting the resistance value of the MTJ element 30. The resistance value is varied by applying a magnetic field to the MTJ element 30. The varied resistance value is read out by the following method.
In the example shown in FIG. 26, a MOSFET 14 is used as a switching element for data read-out. As is shown in FIG. 26, an MTJ element 30 is connected in series to, for example, an N+-type source/drain diffusion layer 13 of the MOSFET 14 in one cell. If the gate electrode 12 of the MOSFET 14, which is a chosen one, is turned on, a current path is formed through the following elements in the named order: a bit line 35, the MTJ element 30, a lower electrode 31, a contact 26, a second wiring 22, a contact 18, a first wiring 17, a contact 16, and source/drain diffusion layer 13. Thus, the resistance value of the MTJ element 30, which is connected to the turned-on MOSFET 14, can be read out.
In the example of FIG. 27, a diode 61 is used as a switching element for data read-out. As is shown in FIG. 27, an MTJ element 30 is connected in series to a diode 61 within one cell. The doide 61 is formed of a P+-type diffusion layer and an N+-type diffusion layer. By adjusting a bias voltage so as to cause a current to flow to the diode 61, which is a chosen one, the resistance value of the MTJ element 30 connected to the diode 61 can be read out. In FIGS. 26 and 27, an element-isolating region of a STI (shallow trench isolation) structure is formed in a P-type semiconductor substrate 11. A word line 22 is formed below the MTJ element 30.
If the resistance value, which has been read out as described above, is low, it is determined that information “1” has been written. If the resistance value is high, it is determined that information “0” has been written.
In the above-mentioned prior-art magnetic memory device, however, there are one MTJ element 30 and one switching element in each cell as shown in FIGS. 26 and 27, so that an memory cell array region occupies a large area within this magnetic memory device.